The present invention relates to a method of making shaped articles from rigid-chain polymers, and more particularly, it relates to a method which utilizes a phase transformation of the rigid-chain polymer in a concentrated solution at elevated temperatures to form a wide variety of shaped articles including bulk components.
The use of organic polymers in the formation of structural materials has been desirable as such polymers can be melt processed at low temperatures, are environmentally stable, and are light in weight. However, such polymers are not suitable for high temperature use applications such as reinforcing fibers.
In recent years, high temperature, high-performance rigid-chain polymers have been developed by incorporating aromatic and heteroaromatic structures into the polymer backbone. Such rigid-chain, or rigid-rod, polymers include poly(p-phenylene benzobisthiazole) (PBT), poly(p-phenylene benzobisoxazole) (PBO) and poly(p-phenylene benzobisimidazole) (PBI), ladder polymers such as poly(imidazoisoquinolines) (BBL), extended-chain polymers such as poly(p-phenylene terephthalamide) (PPTA), poly(2,5(6)benzothiazole) (ABPBT) and poly(2,5(6) benzimidazole) (ABPBI), as well as their functionalized derivatives such as benzocyclobutene-functionalized PBT and PPTA. These polymers have a thermal and thermooxidative stability up to 500.degree. C. When subjected to elevated temperatures, they normally decompose but do not melt.
However, these thermally intractable rigid-chain polymers are soluble in strong acids, such as polyphosphoric acid (PPA), methanesulfonic acid (MSA) and sulfuric acid, as well as organic solvents with Lewis acids. It has been found that when these polymers are dissolved in such acids, the resulting solutions may be extruded and coagulated to form uniaxially oriented fibers and biaxially oriented films. However, while such fibers have excellent tensile properties, they have poor compressive strength due to the fibrillar structure of the fibers, which is weak in lateral interactions and tends to buckle under a compressive load. This poor axial compressive strength has prevented the rigid-chain polymer fibers from being used as reinforcing fibers in more advanced structural composites.
Further, it is difficult to process the thermally intractable rigid-chain polymers into other structural materials such as bulk structural components because the extraction of solvent from such large components is time consuming and often results in shrinkage and deformation of the components.
Accordingly, there is still a need in the art for an improved method of processing thermally intractable rigid-chain polymers into structural components such as fibers and bulk components which exhibit good compressive strength and which do not undergo shrinkage or deformation during formation.